Concrete in Cryogenic Temperature

At temperature
higher than -120F (-110C) concrete shows small increase
in strength. Development of strength depends on ambient temperature of
concrete. The datum temperature of concrete is -120F.

Now can
concrete be exposed to cryogenic temperature? We will find answer of this
question later, at first, we will learn about cryogenic temperature. There have
no well-defined point in temperature scale where refrigeration ends. Cryogenic
begins when refrigeration ends, it is assumed that it begins ≤
-2380F (1500C).

When concrete storage tank is used to contain liquefied natural gas (LNG), concrete is
exposed to cryogenic temperature. The boiling point of LNG is -2600F
(-1620C).

When ambient
temperature of concrete drop down from freezing point (water) to about -3300F
(-2000C), the strength of it found significantly higher than that
kept in room temperature. Compressive strength may be several times more than strength
of concrete in room temperature. But moisture content is the main varying
factor.

The compressive
strength is often found (2~3) times of the strength at room temperature when
concrete is in moist condition while it is being chilled, but in case of air
dried concrete this increase is found much less.

So there has
difference between strength of dry and wet concrete; this is due to ice
formation in hydrated cement paste. Again freezing point of water in gel
depends on pore size; the smaller the pore the lower freezing point of water. The
usual range at which all water adsorbed in pore become frozen lies between -1120~1390F
(-800~950C).

Unlike water,
ice formed in pores with in gel has resistance against stress and frozen
concrete shows extremely low apparent porosity. low We know low porosity offer
higher strength and greater durability (though later one is not true here).

Again compressive
strength of such concrete (actually ice) depends on temperature. Temperature also
controls thermal expansion of ice. Thus behavior of hydrated cement paste is
very complex. If ambient temperature of concrete is not very low, the porosity
of concrete would be low and strength increase would be found very low.

A variation of
compressive strength with temperature of lightweight aggregate concrete both in
moist and air-dry condition are shown below:

Above figure
shows that temperature below -1200C compressive strength increases
very small. This is due to reaching a region of temperature scale where
internal structure of the ice in pores is changed. Actually at -1130C
the hexagonal structure of ice takes forms of orthorhombic structure. Volume
decease of around 20 percent is usually accompanied by this change in
structure.

So, cyclic
variation of temperature around concrete has to be investigated carefully as temperature
gradient is very important in structural design, especially, at this range of temperature.

Now let’s
consider tensile strength of concrete in cryogenic temperature. Tensile
strength is important as liquid gas obviously produce pressure on cryogenic
tank. Regarding tensile strength, it can be conclude that the behavior of
concrete is not identical as that in compressive strength;there have also an increase in tensile
strength but in compared to compressive strength it is smaller.

Following
figure shows variation of tensile strength with temperature. Above two figures correspond
to lightweight aggregate concrete; this type of concrete is considered as cryogenic
tank need good insulation properties of concrete.

xxxxForm figure we
can conclude that tensile strength increase within limit of -70 and
-870C. Now consider normal concrete, the increase in strength is
more pronounced at low temperature than that of lightweight.

When moisture
content is considered, the related term w/c ratio has to be investigated, it is
very interesting to notice that compressive strength increase with moisture
content is not related to water/cement ration.

Modulus of
elasticity of concrete both in moist and air-dry concrete are also changed with
temperature change. This parameter of moist concrete shows a steady increase
with temperature decrease till -1900C. At this temperature air-dry
concrete has modulus of elasticity around 1.65 times that corresponds to room temperature;
for moist concrete the value is about 1.75.